The present invention relates to a recording device using a semiconductor laser which is capable of reproducing a picture such as a photograph having half-tones with a high accuracy.
To intensity modulate a laser beam to record the image of a picture having half-tones, any of (1) a technique of using an ultrasonic optical modulator, (2) a technique of varying the discharge current of a gas laser, and (3) a technique of varying the current of a semiconductor laser may be employed.
The first technique is disadvantageous in that it uses an expensive ultrasonic optical modulator and mechanism for finely adjusting a modulator to the Bragg angle and hence, as a whole, has a high manufacturing cost and intricate construction.
The second technique of varying the discharge current of the gas laser is also disadvantageous in that the modulation frequency is low, several hundred Hertz, and the service life of the laser tube is reduced due to variations of the discharge current.
The third technique of varying the current of the semiconductor laser is disadvantageous in that, since the semiconductor laser has an optical output vs. current characteristic as shown in FIG. 1, the optical output is greatly changed merely by slightly changing the input current thereto, and accordingly it is considerably difficult to record an image having half-tones by controlling the optical output in an analog mode by varying the applied current.
Accordingly, an object of the invention is to provide a laser recorder which can reproduce half-tones with high accuracy.
The invention is intended to improve the quality of a picture which is produced by a laser recorder in which, as disclosed in U.S. Patent Application Ser. No. 214,815 filed Dec. 9, 1980, an input signal is sampled with a sampling pulse signal, a high frequency pulse having a frequency is of at least 10 Hz is generated using the sampling pulse signal, and the number of high frequency pulses which are outputted during a sampling period is controlled and applied to a semiconductor laser.
The term "sampling pulse" or "sampling pulse signal" as herein used is intended to mean a pulse or pulse signal for sampling an input video signal at predetermined time intervals. The frequency of the sampling pulse signal may be selected as desired although it is preferable, in order to reproduce the picture with a high resolution, that the frequency be slightly higher than the maximum frequency of the input video signal. Furthermore, the term "high frequency pulse signal" is intended to mean a pulse signal having a frequency higher than that of the sampling pulse signal preferably a several hundred to several thousand Hertz. These two pulse signals may be generated independently of each other. However, it is desirable that the sampling pulse signal be obtained by frequency-dividing the high frequency pulse signal.
The amount of exposure of each of the picture elements which form a picture is defined by the number of high frequency pulses which are applied to a semiconductor laser according to the level of an input video signal during a sampling period. If the optical energy of the semiconductor laser applied to a photosensitive material corresponding to one high frequency pulse is represented by .DELTA.e, and the number of high frequency pulses which are provided for a picture element according to the level of an input video signal is represented by N, the total optical energy, i.e. the exposure E applied to the picture element is: EQU E=N.multidot..DELTA.e. (1)
The number of high frequency pulses not only corresponds linearly to the input signal, but also may take into account the logarithmic conversion of the input signal and characteristics of a recording material employed, or a predetermined stored input and output characteristic. The term "input signal" as herein used is intended to mean a video signal, which may be either an analog signal or a digital signal.
The relation between the number of pulses and the density of a recorded image where the image is recorded with a semiconductor laser which is controlled by the number of high frequency pulses applied thereto will be described with reference to FIG. 2.
Curve I in FIG. 2 is a characteristic curve of a recording material. More specifically, it is an example of the relation between the logarithmic value of the exposure E and the density D. Curve II in FIG. 2 is an example of the relation between the number N of high frequency pulses outputted and the logarithmic value of the exposure E for the recording material obtained from the number N.
Once a density level is selected in FIG. 2, the corresponding number N of high frequency pulses can be obtained as indicated by the arrows in FIG. 2. If, for instance, the density D is changed from 0.1 to 0.2 in the low density part of FIG. 2, the pulse number N is increased only by about nine pulses. However, if the density D is changed from 1.3 to 1.4 in the high density part of the curve, it is necessary to increase the pulses number N by about 50 pulses.
As is apparent from the above description, in order to reproduce gradations at equal density intervals with a sufficiently high accuracy, the frequency of the high frequency pulse signal must be much higher than that of the sampling pulse signal, for instance higher by several hundred to several thousand times.
The relation of the frequency f.sub.s of the sampling pulse signal, the frequency f.sub.H of the high frequency pulse signal and the maximum pulse number N.sub.max required for the maximum level of the input signal for which the maximum exposure should be provided is defined by the following expression: EQU f.sub.H .gtoreq.N.sub.max .times.f.sub.s. (2)
If it is required to improve the accuracy in reproducing the gradation by making the density intervals smaller or, depending on the characteristics of the photosensitive material such as the maximum gradient .gamma. of the characteristic curve and the range of density D, the maximum pulse number N.sub.max will be much larger than the pulse number N in FIG. 2. As a result, the frequency f.sub.H of the high frequency pulse signal becomes very high. Accordingly, it may be difficult to construct a circuit for implementing this technique.
For instance, if the sampling pulse frequency f.sub.s =100 KHz and the maximum pulse number N.sub.max =500, the corresponding frequency f.sub.H of the high frequency pulse signal is, from expression (2), 50 MHz. In this case, the implementing circuit cannot be constructed with standard TTL logic elements and ECL logic elements must be used with the result that disadvantageously the circuit is considerably expensive.
Another drawback is as follows: If as shown in FIG. 3, the maximum number N.sub.max of high frequency pulses is outputted during the sampling period of a picture element, no light is provided during a period (shaded part in FIG. 3ii) between adjacent pulses as a result of which the utilization factor of the light source is low accordingly making it necessary to increase the intensity of the output light of the light source.
Furthermore, as described in co-pending U.S. Patent application Ser. No. 214,815, when the light beam is outputted with a pulse width corresponding to the number N of high frequency pulses as indicated in FIG. 3iii (hereinafter referred to as "pulse-width" modulation 11 when applicable), the optical utilization factor is approximately doubled while the exposure increment .DELTA.e is correspondingly about twice that for pulse-number modulation (FIG. 3ii). As a result, the resolution of an exposure level for each picture element, i.e. the reproduction density resolution, is unavoidably reduced.
In view of the foregoing, an object of the invention is to provide a laser recorder in which the frequency f.sub.H of the high frequency pulse signal can be reduced to half of that in the conventional device with the reproduction density resolution maintained unchanged, whereby the circuit manufacturing cost is decreased and the utilization factor of the light source is approximately doubled.